8. Mass spectra and IR
Mass spectrometry
Measuring the Mr of an organic molecule
Spectra for C4H10
Mass spectrum for butane
If a molecule is put through a mass spectrometer
it will often break up and give a series of peaks
caused by the fragments. The peak with the
largest m/z, however, will be due to the complete
molecule and will be equal to the Mr of the
molecule. This peak is called the parent ion or
molecular ion
43
Molecular ion
C4H10+
29
58
Fragmentation
Molecular ion formed:
When organic molecules are passed through a mass
spectrometer, it detects both the whole molecule and
fragments of the molecule.
M → [M]+. + e–
The molecule loses an electron and
becomes both an ion and a free radical
Several peaks in the mass spectrum occur due to fragmentation.
The Molecular ion fragments due to covalent bonds breaking: [M]+. → X+ + Y.
This process produces an ion
and a free radical. The ion is
responsible for the peak
Relatively stable ions such as carbocations R+ such as CH3CH2+ and
acylium ions [R-C=O]+ are common. The more stable the ion, the greater
the peak intensity.
The peak with the highest mass/charge ratio will be normally due to the
original molecule that hasn’t fragmented (called the molecular ion) . As
the charge of the ion is +1 the mass/ charge ratio is equal to Mr.
Equation for formation molecular ion
Mass spectrum for propanal
29
CH3CH2CHO → [CH3CH2CHO]+. + e–
m/z 58
[CHO]+
Equations for formation of fragment ions from molecular ions
58
[CH3CH2CHO]+.
Mass spectrum for propanone
The high peak
at 43 due to
stability of acyl
group
43
[CH3CO]+
[CH3COCH3]+.
[CH3CH2CHO]+. → [CHO]+ + .CH2CH3
[CH3CH2CHO]+. → [CH3CH2]+ + .CHO
m/z 29
m/z 29
Equation for formation molecular ion
CH3COCH3 → [CH3COCH3]+. + e–
m/z 58
Equations for formation of fragment ions from molecular ions
[CH3COCH3]+. → [CH3CO]+ +
.
CH3 m/z 43
58
15
It is not possible for propanone to fragment to give a
peak at 29 so the fragmentation patterns can distinguish
between the structural isomers of propanone and
propanal
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Infrared spectroscopy
Complicated spectra can be obtained than
provide information about the types of bonds
present in a molecule
Certain bonds in a molecule absorb infra-red radiation at
characteristic frequencies causing the covalent bonds to
vibrate
ABOVE 1500 cm-1 – “Functional group identification”
BELOW 1500 cm-1 – “Fingerprinting”
Complicated and contains many
signals – picking out functional
group signals difficult.
This part of the spectrum is unique
for every compound, and so can be
used as a "fingerprint".
e.g. C=O 1640 – 1750 cm-1
A computer will compare the IR
spectra against a database of
known pure compounds to identify
the compound
O-H (acid) 2500- 3300 cm-1
Use an IR absorption table provided in
exam to deduce presence or absence
of particular bonds or functional groups
Bond
Wavenumber
C-O
1000-1300
C=O
1640-1750
C-H
2850 -3100
O-H
Carboxylic acids
2500-3300
Very broad
N-H
3200-3500
O-H
Acohols, phenols
3200- 3550
broad
Use spectra to identify particular functional groups e.g. an alcohol from
an absorption peak of the O–H bond, or C=O stretching absorption in
aldehydes and ketones
Spectra for
butanal
O
H3C CH2 CH2 C
2000
1500
C=O
Absorption or trough in between 1640-1750 cm-1 range indicates
presence of C=O bond
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H
Always quote the wave number range from the data sheet
Spectra for
ethanoic acid
O-H absorptions tend
to be broad
O
Absorption or trough in
between 2500-3300 cm-1 range
indicates presence of O-H
bond in an acid
H3C
C
OH
C=O
Absorption or trough in between 16401750 cm-1 range indicates presence of
C=O bond
Molecules which change their polarity as they vibrate can
absorb infrared radiation. E.g. C-H, C=O, O-H
Molecules such as H2, O2 and N2 cannot change their polarity as they vibrate
so can absorb infrared radiation and don’t register on an infra red spectrum
The absorption of infra-red radiation by bonds in this type of spectroscopy is the same absorption
that bonds in CO2, methane and water vapour in the atmosphere do that cause them to be
greenhouse gases.
H2O, CO2, CH4 and NO molecules absorb IR radiation
and are greenhouse gases, whilst O2 and N2 are not.
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